1. Field of Invention
This invention relates generally to electrical stimulation therapy for epilepsy, more specifically to adjunct treatment for partial complex, generalized epilepsy and involuntary movement disorders using an external stimulator coupled to an easily implantable lead, through a disposable patch electrode.
2. Background
Complex partial seizure is a common form of epilepsy, and some 30-40% of patients afflicted with this disorder are not well controlled by medications. Some patients have epileptogenic foci that may be identified and resected; however, many patients remain who have medically resistant seizures not amenable to resective surgery. Stimulation of the vagus nerve has been shown to reduce or abort seizures in experimental models. Early clinical trials have suggested that vagus nerve stimulation has beneficial effects for complex partial seizures and generalized epilepsy in humans. In addition, intermittent vagal stimulation has been relatively safe and well tolerated during the follow-up period available in these groups of patients. The minimal side effects of tingling sensations and brief voice abnormalities have not been distressing.
The vagus nerve provides an easily accessible, peripheral route to modulate central nervous system (CNS) function. Other cranial nerves can be used for the same purpose, but the vagus nerve is preferred because of its easy accessibility. In the human body there are two vagal nerves (VN), the right VN and the left VN. Each vagus nerve is encased in the carotid sheath along with the carotid artery and jugular vein. The innervation of the right and left vagal nerves is different. The innervation of the right vagus nerve is such that stimulating it results in profound bradycardia (slowing of the heart rate). The left vagal nerve has some innervation to the heart, but mostly innervates the visceral organs such as the gastrointestinal tract. It is known that stimulation of the left vagal nerve does not cause any significant deleterious side effects.
Most nerves in the human body are composed of thousands of fibers, of different sizes designated by groups A, B and C, which carry signals to and from the brain. The vagus nerve, for example, may have approximately 100,000 fibers of the three different types, each carrying signals. Each axon (fiber) of that nerve conducts only in one direction, in normal circumstances. The A and B fibers are myelinated (i.e., have a myelin sheath, constituting a substance largely composed of fat), whereas the C fibers are unmyelinated.
A commonly used nomenclature for peripheral nerve fibers, using Roman and Greek letters, is given in the table below,
External Diameter Conduction Velocity Group (.mu.m) (m/sec) Myelinated Fibers A.alpha. or IA 12-20 70-120 A.beta.: IB 10-15 60-80 II 5-15 30-80 A.gamma. 3-8 15-40 A.delta. or III 3-8 10-30 B 1-3 5-15 Unmyelinted fibers C or IV 0.2-1.5 0.5-2.5
The diameters of group A and group B fibers include the thicknesses of the myelin sheaths. Group A is further subdivided into alpha, beta, gamma, and delta fibers in decreasing order of size. There is some overlapping of the diameters of the A, B, and C groups because physiological properties, especially the form of the action potential, are taken into consideration when defining the groups. The smallest fibers (group C) are unmyelinated and have the slowest conduction rate, whereas the myelinted fibers of group B and group A exhibit rates of conduction that progressively increase with diameter. Group B fibers are not present in the nerves of the limbs; they occur in white rami and some cranial nerves.
Compared to unmyelinated fibers, myelinated fibers are typically larger, conduct faster, have very low stimulation thresholds, and exhibit a particular strength-duration curve or respond to a specific pulse width versus amplitude for stimulation. The A and B fibers can be stimulated with relatively narrow pulse widths, from 50 to 200 microseconds (.mu.s), for example. The A fiber conducts slightly faster than the B fiber and has a slightly lower threshold. The C fibers are very small, conduct electrical signals very slowly, and have high stimulation thresholds typically requiring a wider pulse width (300-1,000 .mu.s) and a higher amplitude for activation. Selective stimulation of only A and B fibers is readily accomplished. The requirement of a larger and wider pulse to stimulate the C fibers, however, makes selective stimulation of only C fibers, to the exclusion of the A and B fibers, virtually unachievable inasmuch as the large signal will tend to activate the A and B fibers to some extent as well.
The vagus nerve is composed of somatic and visceral afferents (i.e., inward conducting nerve fibers which convey impulses toward the brain) and efferents (i.e., outward conducting nerve fibers which convey impulses to an effector). Usually, nerve stimulation activates signals in both directions (bi-directionally). It is possible, however, through the use of special electrodes and waveforms, to selectively stimulate a nerve in one direction only (unidirectionally). The vast majority of vagal nerve fibers are C fibers, and a majority are visceral afferents having cell bodies lying in masses or ganglia in the skull. The central projections terminate largely in the nucleus of the solitary tract which sends fibers to various regions of the brain (e.g., the hypothalamus, thalamus, and amygdala).
The basic premise of vagal nerve stimulation for control of seizures is that vagal visceral afferents have a diffuse central nervous system (CNS) projection, and activation of these pathways has a widespread effect on neuronal excitability.
The cervical component of the vagus nerve (10.sup.th cranial nerve) transmits primarily sensory information that is important in the regulation of autonomic activity by the parasympathetic system. General visceral afferents constitute approximately 80% of the fibers of the nerve, and thus it is not surprising that vagal stimulation (VS) can profoundly affect CNS activity. With cell bodies in the nodose ganglion, these affcrents originate from receptors in the heart, aorta, lungs, and gastrointestinal system and project primarily to the nucleus of the solitary tract which extends throughout the length of the medulla oblongata. A small number of fibers pass directly to the spinal trigeminal nucleus and the reticular formation.
As might be predicted from the electrophysiologic studies, the nucleus of the solitary tract has widespread projection to cerebral cortex, basal forebrain, thalamus, hypothalamus, amygdala, hippocampus, dorsal raphe, and cerebellum as shown in FIG. 1 (from Epilepsia, vol.3, suppl.2: 1990, page S2).
Observations on the profound effect of electrical stimulation of the vagus nerve on central nervous system (CNS) activity, extends back to 1930's. In mid-1980s it was suggested that electrical stimulation of the vagus nerve might be effective in preventing seizures. Early studies on the effects of vagal nerve stimulation (VNS) on brain function focused on acute changes in the cortical electroencephalogram (EEG) of anesthetized animals. Investigators found that VNS could temporarily synchronize or desynchronize the electroencephalogram, depending on the level of anesthesia and the frequency or intensity of the vagal stimulus. These observations had suggested that VNS exerted its anticonvulsant effect by desynchronizing cortical electrical activity. However, subsequent clinical investigations have not shown VNS-induced changes in the background EEGs of humans. A study, which used awake and freely moving animals, also showed no VNS-induced changes in background EEG activity. Taken together, the findings from animal study and recent human studies indicate that acute desynchronization of EEG activity is not a prominent feature of VNS when it is administered during physiologic wakefulness and sleep, and does not explain the anticonvulsant effect of VNS.
The mechanism by which vagal stimulation (VS) exerts its influence on seizures is not entirely understood. An early hypotheses had suggested that VS utilizes the relatively specific projection from the nucleus of the solitary track to limbic structures to inhibit partial seizures, particularly those involving cortex, which regulates autonomic activity or visceral sensations such as in temporal lobe epilepsy. Afferent VS at the onset of a partial seizure could abort the seizure in the same way somatosensory stimuli can abort a seizure from the rolandic cortex; however, chronic intermittent stimulation may also produce an alteration in limbic circuitry that outlasts the stimulus and decreases epileptogenesis or limits seizure spread. Support for this hypothesis comes from studies of fos immunoreactivity in the brain of rats in response to VS. Fos is a nuclear protein resulting from expression of early immediate genes in highly active neurons. VS causes a specific fos immunolabeling in amygdala and limbic neocortex, suggesting that the antiepileptic effect may be mediated in these areas. Such activation of genetic mechanisms could account for the apparent sustained antiepileptic effect of intermittent stimulation.
Another possible mechanism that is being explored to explain an antiseizure effect of VS is activation of the brainstem noradrenergic nuclei, lucus ceruleus and A5, which also show fos immunolabeling. Noradrenergic mechanisms are well known to influence seizure activity in genetic epilepsy-prone rats, and the anticonvulsant effects of VS against maximal electroshock seizures can be blocked inactivation of the loc. ceruleus. Woodbury and Woodbury (1990) suggested that VS acts through increasing release of glycine or GABA since seizures induced by both PTZ and strychnine can be blocked by VS. Other neruotransmitter systems may also be implicated since VS increases cerebrospinal fluid homovanilic acid and 5-hydroxyindoleacetate, suggesting modulation of dopaminergic and serotonergic systems. Finally, a nonspecific alteration of activity in the brainstem reticular system with subsequent arousal must be considered.
VS appears to have similar efficacy in both partial and generalized seizures in experimental models and in human epilepsy consistent with a nonspecific effect. Furthermore, the same inhibition of interictal corticalspike activity as seen with VS occurs in animals during electrical stimulation of the midbrain reticular formation or with thermal stimulation of somatosensory nerves in the rat tail. Reduction of experimental generalized spike wave by arousal has also been documented. Similarly, nonspecific afferent stimulation has been well demonstrated in humans to suppress focal spikes, generalized spike waves, and seizures.
VNS may inhibit seizures directly at the level of cerebral cortical neuronal irritability, or at the level of diffuse ascending subcortical projection systems, or both. Thus, VNS is also well suited for the treatment of medication-resistant symptomatic generalized epilepsy (SGE), in which, characteristically both focal and generalized features are found on interictal EEGs and also in clinical seizure types.
U.S. Pat. No. 3,796,221 (flagfors) is directed to controlling the amplitude, duration and frequency of electrical stimulation applied from an externally located transmitter to an implanted receiver by inductively coupling. Electrical circuitry is schematically illustrated for compensating for the variability in the amplitude of the electrical signal available to the receiver because of the shifting of the relative positions of the transmitter-receiver pair. By highlighting the difficulty of delivering consistent pulses, this patent points away from applications such as the current application, where consistent therapy needs to be continuously sustained over a prolonged period of time (24 hours a day for years). The methodology disclosed is focused on circuitry within the receiver, which would not be sufficient when the transmitting coil and receiving coil assume significantly different orientation, which is likely in the current application. The present invention discloses a novel approach for this problem. Additionally, the mode of stimulation in Hagfors patent is "bipolar" whereas in the current patent, the mode of stimulation is "unipolar" i.e. between the tip electrode and case.
U.S. Pat. Nos. 4,702,254, 4,867,164 and 5,025,807 (Zabara) generally disclose animal research and experimentation related to epilepsy and the like and are directed to stimulating the vagas nerve by using pacemaker technology, such as an implantable pulse generator. These patents are based on several key hypotheses, some of which have since been shown to be incorrect. The pacemaker technology concept consists of a stimulating lead connected to a pulse generator (containing the circuity and DC power source) implanted subcutaneously or submuscularly, somewhere in the pectoral or axillary region, with an external personal computer (PC) based programmer. Once the pulse generator is programmed for the patient, the fully functional circuity and power source are fully implanted within the patient's body. In such a system, when the battery is depleted, a surgical procedure is required to disconnect and replace the entire pulse generator (circuitry and power source). These patents neither anticipate practical problems of an inductively coupled system for adjunct therapy of epilepsy, nor suggest solutions to the same for an inductively coupled system for adjunct therapy of partial complex or generalized epilepsy. FIG. 4 in all three above Zabara patents show the stimulation electrode around the right vagus nerve. It is well known that stimulation of right vagus can lead to profound bradycardia (slowing of the heart rate), an unwanted complication.
U.S. Pat. No. 5,752,979 (Benabid) is directed to a method of controlling epilepsy with stimulation directly into the brain, utilizing an implantable generator. More specifically, Benabid discloses electrically stimulating the external segment of the globus palliaus nucleus of the brain causing increased excitation, thereby increasing inhibition of neural activity in the subthalanic nucleus and reducing excitatory input to the substantia nigra leading to a reduction in the occurrence of seizures.
U.S. Pat. No. 5,540,734 (Zabara) is directed to stimulation of one or both of a patient's trigeminal and glossopharyngeal nerve utilizing an implanted pulse generator.
U.S. Pat. No. 5,031,618 (Mullett) discloses a position sensor for chronically implanted neuro stimulator for stimulating the spinal cord. The position sensor, located in a chronically implanted programmable spinal cord stimulator, modulates the stimulation signals depending on whether the patient is erect or supine.
U.S. Pat. No. 4,573,481 (Bullara) is directed to an implantable helical electrode assembly configured to fit around a nerve. The individual flexible ribbon electrodes are each partially embedded in a portion of the peripheral surface of a helically formed dielectric support matrix.
U.S. Pat. No. 3,760,812 (Timm et al.) discloses nerve stimulation electrodes that include a pair of parallel spaced apart helically wound conductors maintained in this configuration.
U.S. Pat. No. 4,979,511 (Terry) discloses a flexible, helical electrode structure with an improved connector for attaching the lead wires to the nerve bundle to minimize damage.
An implantable pulse generator and lead with a PC based external programmer is advantageous for cardiac pacing applications for several reasons, including:
1) A cardiac pacemaker must sense the intrinsic activity of the heart, because cardiac pacemakers deliver electrical output primarily during the brief periods when patients either have pauses in their intrinsic cardiac activity or during those periods of time when the heart rate drops (bradycardia) below a certain pre-programmed level. Therefore, for most of the time, in majority of patients, the cardiac pacemaker "sits" quietly monitoring the patient's intrinsic cardiac activity.
2) The stimulation frequency for cardiac pacing is typically close to 1 Hz, as opposed to approximately 20 Hz or higher, typically used in nerve stimulation applications.
3) Patients who require cardiac pacemaker support are typically in their 60's, 70's or 80's years of age.
The combined effect of these three factors is that the battery in a pacemaker can have a life of 10-15 years. Most patients in whom a pacemaker is indicated are implanted only once, with perhaps one surgical pulse generator replacement.
In contrast, patients with partial complex epilepsy or generalized epilepsy in whom electrical stimulation is beneficial are much younger as a group, typically ranging from 12 to 45 years in age. Also, stimulation frequency is typically 20 Hz or higher, and the total stimulation time per day is much longer than for cardiac pacemakers. As a result, battery drain is typically much higher for nerve stimulation applications than for cardiac pacemakers.
The net result of these factors is that the battery will not last nearly as long as in cardiac pacemakers. Because the indicated patient population is also much younger, the expense and impact of surgical generator replacement will become significant, and detract from the appeal of this therapy. In fact, it has been reported in the medical literature that the battery life can be as short as one and half years for implantable nerve stimulator. (R.S McLachlan, p. 233).
There are several other advantages of the present inductively coupled system.
1) The hardware components implanted in the body are much less. This is advantageous for the patient in terms of patient comfort, and it decreases the chances of the hardware getting infected in the body. Typically, when an implantable system gets infected in the body, it cannot be easily treated with antibiotics and eventually the whole implanted system has to be explanted.
2) Because the power source is external, the physician can use stimulation sequences that are more effective and more demanding on the power supply, such as longer "on" time.
3) With the controlling circuitry being external, the physician and the patient may easily select from a number of predetermined programs, override a program, manually operate the device or even modify the predetermined programs.
4) The external inductively-coupled nerve stimulation (EINS) system is quicker and easier to implant.
5) The external pulse generator does not need to be monitored for "End-of-Life" (EOL) like the implantable system, thus resulting in cost saving and convenience.
6) The EINS system can be manufactured at a significantly lower cost of an implantable pulse generator and programmer system, providing the patient and medical establishment with cost effective therapies.
7) The EINS system makes it more convenient for the patient or caretaker to turn the device on during an "Aura" that sometimes precedes the seizures. Also, because programming the device is much simpler, the patient or caretaker may reprogram the device at night time by simply pressing one or two buttons to improve patient comfort.
8) Occasionally, an individual responds adversely to an implanted medical device and the implanted hardware must be removed. In such a case, a patient having the EINS systems has less implanted hardware to be removed and the cost of the pulse generator does not become a factor.
In the conventional manner of implanting, a cervical incision is made above the clavicle, and another infraclavicular incision is made in the deltapectoral region for the implantable stimulus generator pocket. To tunnel the lead to the cervical incision, a shunt-passing tool is passed from the cervical incision to the generator pocket, where the electrode is attached to the shunt-passing tool and the electrode is then "pulled" back to the cervical incision for attachment to the nerve. This standard technique has the disadvantage that it is time consuming and it tends to create an open space in the subcutaneous tissue. Post surgically the body will fill up this space with serous fluid, which can be undesirable.
To make the subcutaneous tunneling simpler and to avoid possible complication, one form of the implantable lead body is designed with a hollow lumen to aid in implanting. In this embodiment, a special tunneling tool slides into a hollow lumen. After the cervical and infraclavicular incisions are made, the tunneling tool and lead are simply "pushed" to the cervical incision and the tunneling tool is pulled out. Since the tunneling tool is inside the lead, no extra subcutaneous space is created around the lead, as the lead is pushed. This promotes better healing post-surgically.
The apparatus and methods disclosed herein also may be appropriate for the treatment of other conditions, as disclosed in co-pending applications filed on Oct. 26, 1998, entitled APPARATUS AND METHOD FOR ADJUNCT (ADD-ON) THERAPY OF DEMENTIA AND ALZHEIMER'S DISEASE UTILIZING AN IMPLANTABLE LEAD AND AN EXTERNAL STIMULATOR and APPARATUS AND METHOD FOR ADJUNCT (ADD-ON) THERAPY FOR PAIN SYNDROMES UTILIZING AN IMPLANTABLE LEAD AND AN EXTERNAL STIMULATOR, the disclosures of which are incorporated herein by reference.